In recognition of the importance of needle sharing as a causal factor in acquiring viral infections and later developing cancer, a collaborative project has been undertaken with Dr. Steffanie Strathdee and Homayoon Farzadegan at the Johns Hopkins University - School of Hygiene and Public Health, who evaluate the Baltimore Needle Exchange Program (NEP), and Dr. David Vlahov at the New York Academy of Medicine. Needle sharing is a common means by which intravenous drug users (IVDUs) acquire viral infections, most commonly with hepatitis B, hepatitis C and HIV-1. The hepatitis viruses are causative agents in liver cirrhosis and hepatocellular carcinoma and HIV-1, as the causative agent of AIDS, has cancer outcomes of lymphomas, Kaposi's sarcoma and other rare cancers. In the initial experiments, syringe content samples were genotyped and analyzed for the short tandem repeat (STR) locus D6S502 using customized methods. Experiments were conducted blind to whether or not syringe contents represented single or multi-person use on a panel of reference samples. Following polymerase chain reaction amplification and gel electrophoresis, electropherograms were generated to ascertain the pattern of alleles present in each individual needle wash. Using two-by-two tables, sensitivity and specificity were 68% and 100%, respectively upon re-analysis of the data. The extent of agreement over and above chance, as indicated by calculation of the Kappa coefficient, indicated good agreement. We are confident that the sensitivity can be improved beyond 90% through the analysis of multiple loci. The most recent work has focused on developing a highly sensitive assay for multiplex determination of pentanucleotide STRs. Current assays are able to detect four loci in multiplex with 0.1 ng or less of DNA. These methods were developed to test several hypotheses related to needle sharing and viral infection. One, self-reports of multi-person use of syringes underestimate true levels. Two, multi-person use of syringes has significantly declined over time since the NEP was introduced. Three, NEP syringes acquired by secondary exchange will have circulated longer and will have higher rates of multi-person syringe use, compared to NEP syringes acquired by primary exchange. Four, syringe sharing can identify high-risk seronegatives for our HIV-1 genetic epidemiology studies aimed at identifying infection resistance genes.